Layered antennas, also known as flat plate antennas comprise a ground plane having an aperture or an array of apertures, a dielectric spacer, a feed probe network (typically copper tracks printed on a thin dielectric such as polyester) a second dielectric spacer and a second apertured ground plane.
Such an antenna is known from GB 2261554 (Northern Telecom). The ground planes may be shaped about an axis of the apertures to improve azimuth beamwidth GB 95 02528.4 (Northern Telecom). The array may be dual polarised, with e.g. a first set of feed probes in a vertical direction and with a second set of feed probes perpendicular to the first set of feed probes. Advantageously the dual polarised arrangement is defined by two layered antennas separated by a common ground plane, to reduce coupling between the polarisations.
A particular problem encountered with dual-polarised linear array antennas is that the azimuthal beamwidth of the two polarisations is dissimilar. Typically the axis of the array is arranged in a vertical orientation and the height and power of the array determine the size of each aperture. Such an optimum will have an azimuthal beamwidth of e.g. 75.degree.-90.degree.. Ideally the beamwidth of the two polarisation overlap. Under such circumstances, however, the edge portions of the ground plane--which cover the feed network for the probes--modify the beamwidth to e.g. 60.degree. resulting in a requirement for either a) more vertical arrays or b) a modification of the width of each aperture so that it is unacceptably wide.
The present invention seeks to provide an antenna which overcomes or reduces the aforementioned problems.